When achieving communication between communication satellites and world-wide users of a global telecommunication system, accurate knowledge of a location of each user and each satellite is important to establishing and maintaining a communication link. A subscriber unit which combines voice/data communication and accurate, location self-determination has several advantages as described in prior art. Applications of such a subscriber unit in a system include asset management as described in the prior art. Prior art patents also describe applications such as the coordination of activities of search and rescue and various military operations.
Since communications between a subscriber unit and a group of satellites can involve frequent hand-offs between cells created by the individual satellites and between adjacent satellites, knowledge of accurate location data for both the subscriber unit and the satellites can be used to efficiently determine an appropriate hand-off strategy. Accurate location data for a subscriber unit also aids in the acquisition process by permitting precise correction for Doppler and reference frequency induced errors. In addition, accurate subscriber unit location data allows a telecommunication system operating in a global market to conform to diverse rules and procedures which can be imposed by diverse political entities within whose jurisdiction a telecommunication system can operate.
The more accurate the location data, the better. More accurate data allow a telecommunication system to better ascertain when subscriber units cross from one jurisdiction into another. However, costs generally increase in proportion to the accuracy of the location data, and an intense need to keep the costs as low as possible and the revenues as high as possible exists. One cost which is of particular concern is the amount of communication resources which are consumed in maintaining current location data. As more resources are consumed in maintaining current location data, fewer resources are available for use by communication service subscribers and for generating revenues.
A GPS (Global Positioning System) receiver can achieve great accuracy in self-determining location in a short period of time if it has current ephemeris data and almanac data and has not moved beyond 500 to 1000 km from its last location. Generally, a GPS receiver is required to receive the latest ephemeris data from the satellite and so the time to first location fix is dominated by the time taken by the GPS receiver to receive the data. A GPS location acquisition process, in general, includes the following steps:
1) Prediction of GPS satellite visibility/Doppler at the current time; PA1 2) Sequential detection of those satellites thought to be visible. The detection process switches to a sky search on all the satellites if no acquisition is achieved; PA1 3) Acquisition process hands off to track process; PA1 4) Track process includes acquiring message synchronization, and collecting time from a GPS broadcast message; PA1 5) Track process also includes collecting ephemeris data; and PA1 6) Track process continues with the collection of almanac data.
When GPS receivers are turned on from a warm start, a receiver must find appropriate satellites and then collect current ephemeris from the GPS satellites. This data could be transmitted at 50 bits per second, for example, and could take 30 seconds to collect from each of the satellites that are tracked.
If a GPS receiver is doing a cold start or a GPS receiver has moved a great distance around the earth, then the receiver must find the satellites in view without any apriori knowledge. This complete search can take minutes. After satellites are acquired, the receiver then must collect a complete almanac from the GPS satellite, which includes information on the current satellites' orbits and health.
Also, if exact location is not required, then a GPS receiver does not need ephemeris data, and almanac data by itself can be used to estimate satellite location. This results in a loss of accuracy depending on the age of the almanac. A benefit of using almanac data only is that the ephemeris collections can be bypassed and the access time is shortened. However, provisions to update the almanac data must be provided eventually. A GPS receiver takes about 12.5 minutes to accomplish this task and requires a continuous track on at least one GPS satellite. Bit errors and blockages can cause GPS receivers to use multiple 12.5 minute intervals and this can lead to excessive battery drain. Subscriber units are often battery operated, and an excessive battery drain is not acceptable.
Accordingly, there is a significant need for a geo-location subscriber unit to obtain and use positioning information from a global communication satellite system to decrease the time that a subscriber unit located anywhere proximate to the surface of the earth takes to self-determine accurate location data using geo-location information supplied by a global positioning system.